Purpose :
Despite advances in our understanding of the pathophysiology of diabetic macular edema (DME), much remains unknown. The aim of this study was to further understand protein expression in DME and identify new biomarkers through analysis of aqueous humor (AH) using Liquid Chromatography/Tandem Mass Spectrometry (LC-MS/MS).

Methods :
After institutional review board (IRB) approval was obtained, AH samples were obtained at the beginning of cataract surgery from patients with diabetes without retinopathy (control group) and patients with DME (DME group). Aqueous samples were analyzed using LC-MS/MS and relative protein abundances were determined by spectral counting. T-test followed by Hotchberg-Benjamini correction for multiple comparison was used for statistical analyses to determine significance in the differential expression of proteins between control and DME groups. Proteins were classified into functional groups using Scaffold 4.0.

Results :
AH samples from 9 control and 3 DME patients were collected and analyzed. 188 proteins with probability scores of at least 99% were identified. Two proteins were detected at significantly higher levels in the DME samples: cytoplasmic actin (3.4 fold change, P<0.05) and haptoglobin preprotein isoform 2 (3.3 fold change, P<0.01). Cytoplasmic actin is involved in the actin cytoskeleton and intracellular transport and haptoglobin is involved in complement activation, proteolysis and lipid transport.

Conclusions :
To our knowledge, this is the first report of cytoplasmic actin and haptoglobin preprotein isoform 2 found to be significantly elevated in DME. Increased levels of actin, a cytoskeletal protein of pericytes and endothelial cells, may be an indicator of blood-retinal barrier dysfunction and increased levels of haptoglobin, an acute-phase protein, may be related to cellular stress. These newly identified proteins could be novel biomarkers and may play a role in the pathogenesis of DME.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.